Television camera having a thick birefractive plate before the light-splitting system

ABSTRACT

A TELEVISION CAMERA EMPLOYING A DICHROIC MIRROR LIGHTSPLITTING SYSTEM AND A BIREFRACTIVE PLATE BETWEEN THE DICHROIC MIRROR AND AN OBJECTIVE LENS FOR PROJECTING AN IMAGE OF A SCENCE IN THE CAMERA TUBES. THE BIREFRACTIVE PLATE IS POSITIONED BETWEEN THE OBJECTIVE LENS AND THE DICHROIC MIRRO AND HAS A THICKNESS SUFFICIENT TO MODIFY POLARIZED LIGHT FROM THE SCENE BEFORE IT IS INCIDENT ON THE DICHROIC MIRROR.

Feb. 16, 1971 H. DE LANG 3,563,635

TELEVISION CAMERA HAVING A THICK BIREFRACTIVE PLATE BEFORE THELIGHT-SPLITTING SYSTEM Original Filed Dec. 23, 1966 INVENTOR. HENDRIK DeLang United States Patent Ofice US. Cl. 350-171 2 Claims ABSTRACT OF THEDISCLOSURE A television camera employing a dichroic mirrorlightsplitting system and a birefractive plate between the dichroicmirror and an objective lens for projecting an image of a scene in thecamera tubes. The birefractive plate is positioned between the objectivelens and the dichroic mirror and has a thickness suflicient to modifypolarized light from the scene before it is incident on the dichroicmirror.

This application is a continuation of application Ser. No. 604,314,filed Dec. 23, 1966, now abandoned.

This invention relates to television cameras including a light-splittingsystem in the light path between the objective lens and the camera tube,said system comprising at least one dichroic mirror.

A dichroic mirror is built up of a plurality of mirrors havingalternately high and low indices of refraction. The thickness of eachlayer is chosen to be equal to one quarter of the central wave-length ofthe selected frequency band which has to be reflected by the mirror. Asa result of this choice, radiation in the desired band of frequencies isreflected and radiation outside this band is transmitted.

A dichroic mirror has the property that the coefficients of reflectionand transmission for light which is polarized in the plane of incidenceof the mirror, that is to say light at which the electrical vectoroscillates in the plane of incidence, difler from those for light whichis polarized at right angles to the plane of incidence.

If the light is not polarized (natural light) the mirror has meancoefficients of reflection and transmission which can be calculated. If,however, the light is polarized, these coefficients differ from the meanvalues and color shifts occur. These color shifts reach extreme valuesif the beam incident on the mirror is linearly polarized either in theplane of incidence or at right angles to the plane of incidence of themirror. The incident light is often polarized indeed, since reflectionswhich occur in the object space (the scene to be picked up) wholly orsubstantially under Brewster conditions are substantially unavoidable.Reflection under Brewster conditions is referred to if the reflected andtransmitted light beams of a beam incident on the boundary layer of twomedia are at right angles to one another.

An object of the invention is to avoid the aforesaid color shifts.Further objects of the invention will appear as the specificationprogresses.

In accordance with the invention, a thick bi-refractive 3,563,635Patented Feb. 16, 1971 plate is included in the light path between theobjective lens and the light splitting system. The angle made betweenthe slow or fast axis of the plate and the plane of incidence of thedichroic mirror is preferably approximately 45".

The invention is bottomed on the discovery that the wavelengths forwhich the phase difference between the two principal waves of abi-refractive plate, except a whole multiple of 2ahas the same value,lie closer to one another as the plate is thicker. It is to beunderstood that the two principal waves of a bi-refractive plate are theso-called fast wave and the slow wave.

It has been found that, if the incident light has a spectraldistribution of a continuous character, .the distribution of the lightenergy over the components polarized at right angles to the plane ofincidence and in the plane of incidence may be made sufficiently uniformif the double refraction is at least a few wavelengths. This holds goodirrespectively of the state of polarization of the incident light, whichcan and may be polarized either elliptically, or circularly, orlinearly.

Preferably, the double refraction is chosen to be much greater than afew wave-lengths. First, the operation is thus improved still further,and second, technical advantages result therefrom, since the themanufacture is simplified as the biretractive plate is thicker. A quartzplate of 5 mm. thick, cut in parallel with the main axis, has a doublerefraction equal to I00 wave-lengths of light having a wave-length of0.5 micron (green light).

The invention will now be described with reference to the accompanyingdrawing, showing one embodiment of a color television camera accordingto the invention.

The light originating from the scene impinges through an objective lens1 on the light splitting system comprising dichroic mirrors 2 and 3. Thedichroic mirror 2 reflects the red component of the light and transmitsthe blue and green components. The dichroic mirror 3 reflects the bluecomponent and transmits the green one. The camera tubes 4, 5 and 6 thusreceive red, green and blue light, respectively.

A thick bi-refractive plate 7 is arranged between the Objective lens 1and the light splitting system 2, 3 so that the principal section of theplate is at an angle of 45 with the plane of the dichroic mirror 2.

The state of polarization of the polarized light originating from thescene and striking the plate 7 is modified by this plate. Thismodification greatly depends. upon the wave-length of the light with theresult that in the spectral region of the light employed, the lightenergy is on the average distributed substantially uniformly over thevibrations at right angle to the plane of incidence and in the plane ofincidence.

As previously mentioned, the camera according to the invention affordsthe advantage that its operation is substantially independent of thestate of polarization of the incident light. This advantage does nothold good if a ./4 plate were used. Such a plate is effective only forlinearly polarized light of one determined wave-length. For incidentlight which is polarized circularly, the effect of a )./4 plate is evenopposite to what is desired.

I claim:

1. In a television camera, a light-splitting system in the light pathbetween an objective lens and a camera tube, said system comprising atleast one dichroic mirror and a birefractive plate capable of modifyingthe state of 4 polarized light received from a scene in the light pathOTHER REFERENCES between the i lens and i Smartt et al., Birefringenceof Quartz and Calcite,-

2. A television camera as claimed 1n claim 1, H1 which 49(7) 710 712July (1959 the angle made between the slow or fast axis of the plateshurcliff at a] Polarized Light 4249 5443 (1964) and the plane ofincidence of the dichroic mirror is substantially 45". DAVID SCHONBERG,Primary Examiner Refmnm I. w. LEONARD, Assistant Examiner UNITED sTATEsPATENTS 2,182,142 12/1939 Ball et a1. 3511-173 CL 2,441,049 5/1948 West350-157 10 9s-12.2;17s s.4;sso 1s7 2,809,570 10/1957 Dearing et a].95---12.2

22 93" UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,563 635 Dated February 16, 1971 Inventor(s) HENDRIK DE LANG It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

[ Column 2, lines 57 and 60, change "7\/4" to Column 2, and Column 3,rewrite Claims 1 and 2 as Claim 1 In a television camera, alight-splitting system in the light path between an objective lens and acamera tube, said system comprising at least one dichroic mirror havinga given plane of incidence and a thick birefractive plate in the lightpath between the objective lens and the mirror, the thickness of thebirefractive plate being sufficient to provide double refraction muchgreater than a few wave-lenghts of radiation incident on the dichroic m:said birefractive plate having a slow and a fast axis one of which formsan angle of approximately 45 with the plane of incidence of the dichroicmirror and both of which are substantially perpendicular to the lightpath.

Signed and sealed this 10th day of August 197 (SEAL) Attest: L

EDWARD M. FLETCHER,JR. WILLIAM E SCHUYLER,

Attesting Officer Commissioner of Paten

